The present invention relates generally to orthopaedic surgery. More particularly, the present invention relates to an arthroplasty plate. Specifically, the present invention relates to an arthroplasty plate for repairing fractures of an end of a long bone, which comprises a convex articulation surface forming part of a ball and socket joint. The present invention is particularly suited to the repair of humeral head fractures, although it is not limited to this application.
A natural shoulder joint may undergo degenerative changes for a variety of reasons, for instance arthritis. Alternatively, a shoulder joint may be fractured or otherwise damaged by an external force. When a shoulder joint is sufficiently degenerated or damaged it may be necessary to replace the natural joint with a prosthetic shoulder joint. Conventional shoulder prostheses comprise a humeral prosthesis, optionally with a glenoid prosthesis. For total or reverse arthroplasty a glenoid component is implanted, however for hemi-arthroplasty the humeral component articulates against the natural glenoid cavity.
When the upper extremity of the humerus is fractured, the humerus generally breaks into several pieces, typically three or four. In particular, for “four part fractures” the humeral head splits off at the level of the anatomical neck, the greater and lesser tuberosities are separated from the humerus shaft below the tuberosities and the tuberosities are separated from one another along the bicipetal groove. As there is no longer a blood supply to the humeral head necrosis may begin. For repair of a four-part fracture, or a three part fracture where the blood supply to the humeral head is damaged, the humeral head is replaced, and the greater and lesser tuberosities are reattached to the humeral shaft.
Typically, the humeral component of a shoulder prosthesis comprises a stem for insertion into a bore extending into the intramedullary canal of the humerus, generally along the longitudinal axis of the bone, and an articulation component, which may be a convex bearing head in the case of total arthroplasty or a concave cup in the case of reverse arthroplasty. Typically, the articulation component is coupled to a neck portion of the stem, which extends from the intramedullary canal at an inclined angle relative to the longitudinal axis of a distal portion of the stem in order to recreate the anatomy of the natural joint. Commonly, humeral stem prostheses are secured in position within the intramedullary bore using bone cement. Alternatively, the stem may be coated with a material which encourages bone growth to hold the stem in position, such as Porocoat® or hydroxyapatite. One such humeral stem prosthesis is commercially available from DePuy Orthopaedics, Inc under the trademark Global FX.
For humeral stem prostheses the fractured proximal humerus may fail to heal correctly or healing may be prolonged due to vascular trauma caused by the original injury. Removal of cancellous bone in the intramedullary canal for insertion of the stem, and the heat generated by the cementation, can cause considerable injury to the endosteum of the canal wall. Resulting necrosis of the bone vascularisation inhibits healing of the bone and may even lead to death and resorption of portions of the proximal humerus, especially the greater and lesser tuberosities. Studies have shown that up to one in four humeral stem implants for the repair of humeral fractures have poor results in terms of migration of the tuberosities.
As an alternative to humeral stem implants it is known to repair some types of proximal humeral fracture using an intramedullary nail extending along a bore formed within the intramedullary canal. Screws pass from outside of the humerus, through holes formed within the nail and into the humeral head and the tuberosities. This type of fixation also suffers from vascular damage in the intramedullary canal. Furthermore, a significant drawback is that because the humeral head is not replaced the nail must be inserted through a hole formed in the articular cartilage of the humeral head. A plug must be inserted into the hole to restore the bearing surface.
As a further alternative it is known to repair proximal humeral fractures where the humeral head is relatively undamaged and appears to have an adequate blood flow using a fixation plate attached to the external surface of the proximal humerus. The fixation plate is attached to the lateral surface of the proximal humerus with locking screws. Screws or pegs extend from a head portion of the fixation plate into the humeral head. One such fixation plate is commercially available from DePuy Orthopaedics, Inc under the trademark S3 Shoulder Plate.
For humeral prostheses in which the natural humeral head is replaced with an articulation component, it is important to ensure that the bearing surface is correctly positioned to recreate the natural anatomy of the joint. One option is to provide a range of articulation components each having a different thickness to adjust the height of the bearing surface relative to the humerus. Alternatively, the depth of implantation of the stem into the intramedullary canal may be adjusted. However, it may be difficult to achieve fine control of the position of the bearing head.
For the repair of fractures of the femoral neck it is known to provide a form of prosthesis generally known as a thrust plate prosthesis. One such form of thrust plate prosthesis is disclosed within PCT patent publication WO-2007/024980-A2. The prosthesis comprises a plate portion to be attached to the lateral external surface of the proximal femur. A femoral neck portion comprises an articulation component in the form of a ball coupled to a thrust plate arranged to be seated upon the resected neck of the femur. A tie rod extends from the lateral bone plate to the thrust plate and draws the ball head towards the lateral plate to secure the prosthesis to the bone. The tie rod is tensioned to pre-stress the bone. Under joint loading, force is transferred from the thrust plate to cortical bone within the femoral neck. Alternatively, if the tie rod is not tensioned to compress the femoral neck then force is transferred through the tie rod to the plate. Consequently, some thrust plate prostheses are provided with elongated plate portions to transfer load away from the femoral head and in turn transfer load to intact bone.
It is an object of embodiments of the present invention to obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere.
In particular, it is an object of embodiments of the present invention to provide a humeral reconstruction prosthesis for the surgical repair of a four-part humeral head splitting fracture, or a three-parts humeral head fracture where the blood supply to the head is interrupted. It is a further object of certain embodiments of the present invention to provide a prosthesis which may be implanted without significant vascular trauma of the proximal humerus, thereby promoting faster healing of the fracture.